It appears that instant checkout has been deprecated by playfab since 2017. I believe it would be nice for playfab to update it to their newer standards and let us integrate this system with our games. Researching a little bit, it appears that benefits include players directly entering credit/debit card info without having to create a paypal account (which is super sweet).
Also, I'm just gonna sneak in the long asked support for PayPal sandbox down here.